The goal is to allow Inbox to send MDN without requiring them to forge the message.
For that we need to to add a new argument to setMessages method (for the moment the migration to MessageSubmission is out of scope). We already have create / update / destroy.
Here we add:
sendMDN: String[MDN]|null a map of creation id (an arbitrary string set by the client) to MDN objects (see below for a detailed description).
And here is for the MDN object:
messageId: String id of the message which we want to send an MDN for
subject: String human readable summary of the MDN (eg. "Read: subject of the original message")
textBody: String human readable explanation of the MDN
reportingUA: String will be used as is as the reporting-UA field
actionMode: "manual-action" | "automatic-action"
sendingMode: "MDN-sent-manually" | "MDN-sent-automatically"
type: "displayed" | "deleted" | "dispatched" | "processed"
Note that here messageId is the JMAP Message Id, not the header one.
In case of error, the `creationId` used should be returned in a `MDNNotSent` property.